1. Field of the Invention
This invention relates to apparatus for telemetering singals in an electronic circuit over long distances and more particularly to digital data-telemetry links utilizing an encoder at the data transmitting end of a system, a decoder at the receiving end, and a cable transmission link connected therebetween.
2. Description of the Prior Art
Every since man began to extract oil, gas and other minerals from beneath the surface of the earth on a commercial basis, there has been a need for determining the environmental characteristics existing at various depths in a borehole. In the earliest days of petroleum and mineral exploration, such boreholes were not excessively deep, and the required information concerning the environment was not particularly complex. As a result, the logging instruments used to acquire such information were basically simple and did not have to operate in particularly hostile surroundings.
However, as shallow petroleum and mineral deposits were exploited, boreholes became ever deeper and more expensive, requiring not only an increase in the sophistication of drilling techniques but also improved knowledge with increasing detail and reliability about the rock formations through which the borehole passed. Furthermore, the increased scarcity and value of petroleum and mineral products often led to secondary-recovery projects, further adding to the requirement for more detailed knowledge of the rock formations and the fluids contained in them. This added information often had to be obtained in an increasingly hostile environment characterized by high temperatures and pressures and by long cables for data transmission and control of the subsurface equipment.
As logging cables became longer and correspondingly hotter, their losses at both high and low frequencies became more severe. As a result, signals transmitted over long cables were attenuated and otherwise distorted, causing amplitude measurements at the surface to have reduced accuracy owing to large unstable attenuation ratios and contamination by noise. This problem was particularly severe whenever the amplitude of relatively narrow pulses was important, because the high-frequency attenuation and distortion of long cables tended to be considerably larger than those at low frequencies. These attenuation and distortion effects were also objectionable for shorter cables whenever newer sophisticated subsurface instruments required more precise amplitude analysis than that required by older instruments.
Limited cable bandwidth also caused problems even when standard-amplitude pulses were used and only their average rate of arrival was important. In that case the limited cable bandwidth forced the transmission of pulses with widths considerably longer than that required by detectors and signal-processing electronics alone, thus often providing the limiting factor with regard to pulse rates and dynamic range in frequency. Furthermore, the direct transmission of pulses with random spacings caused problems involving the accidental occurrence of two pulses nearly simultaneously within the restricted response time of the cable, limiting the permissible average pulse rates even beyond that limit required for signals with fixed minimum pulse spacings. This pulse-pileup problem became more severe whenever pulse-amplitude information was desired, adding to the errors from attenuation, distortion and noise.
One solution to this problem known in the prior art involved preventing a second pulse from being sent up the cable until a first pulse had sufficient time to decay to a negligible level. Although this fixed-dead-time technique avoided amplitude and count-rate distortions from pulse pileup, it limited the rate at which information could be transmitted to the surface and sometimes resulted in unacceptable statistical variations. The presence of such fluctuations, in turn, limited the speed with which the formations could be logged, and in extreme cases they have actually forced the logging tool to remain stationary at discrete points instead of moving constantly to provide a continuous record.
For the case where only average pulse repetition rates were important, several other partial solutions to the pulse-pileup problem existed in the prior art. Sometimes a digital counter reduced the raw counting rates to a level consistent with the cable bandwidth. Although this technique did not directly increase the statistical variations for random signals, it did add errors at low counting rates where the lost data contained in the state of the prescaler were significant. Making the length of the prescaler controllable from the surface alleviated this difficulty by adapting the prescaler to the measured counting rate, but the required two-way communications link and its associated complexity were seldom justified. Alternatively, de-randomizing scalers which provided an output frequency which was a short-term average of the input frequency avoided much of the pile-up problem caused by random pulse spacings, but still the upper operating frequency remained severely limited by the restrictive cable bandwidth.
A second class of problems arose because of the need to transmit information from several data sources. For example, in a logging tool for correlating formation parameters with pipe position as defined by counting pipe (casing) collars, a neutron source together with gamma-ray and neutron detectors has often been used in the prior art. Including the need to detect the casing collars, such a tool already has three data sources. Monitoring other parameters indicative of proper tool operation, such as temperature, or indicative of further formation properties, such as natural gamma radiation, further increased the need for handling multiple data sources.
This problem became even more severe for more sophisticated tools such as those employing neutron generators capable of being rapidly pulsed on and off. Even though the interaction of neutrons with the material surrounding the tool was very complex, the limited capacity of prior-art data-transmission systems forced the specialization of such tools, allowing them to observe and record only a small fraction of the parameters characterizing the neutron interaction. As a result, the prior art contained neutron-lifetime tools, porosity tools, chlorine logs, shale indicators, aluminum-activation logs, carbon-oxygen and calcium-silicon logs, sodium logs, magnesium logs, and devices for the detection of uranium. However, no one tool performed more than a few of these functions, whereas in a single hole many such parameters were important and failure to measure some of them sometimes led to an incorrect interpretation of the ones which were measured.
One prior-art technique for increasing the capacity of the data link involved encoding the pulses produced by one data source as positive pulses and those produced by another source as negative pulses and then using this polarity difference at the surface to identify the data source. Not only did this technique have the obvious disadvantage of there being only two polarities and thus only two permissible data sources, but also the limited cable bandwidth and the networks used to couple the pulses to the cable conspired to produce pulse ringing and undershoots, which sometimes confused positive- and negative-signal information. As a result, careful adjustment by trained personnel was often required to obtain even marginally acceptable performance. In addition, the pulse-pileup problem became worse because now two data sources were sharing the limited-bandwidth cable, and they could interfere with each other as well amongst themselves.
Another prior-art method for increasing the information capacity of logging cable without the above disadvantages involved the use of multi-conductor cable, which sometimes was driven as a high-frequency balanced transmission line. However, multi-conductor logging cable was not only expensive, but also it was still severely restricted in bandwidth in long lengths and possessed deleterious interactions between adjacent conductors. Also, because of its large diameter, it was difficult to use in small-diameter tubing or in deep boreholes and often had problems in high-pressure wells, particularly at the pressure interface at the top of the borehole.
Thus, the use of a coaxial monocable with a single center conductor shielded by a load-bearing outer wall remained essential for many logging tools. This approach required that the single center conductor provided operating power for the subsurface equipment as well as a data link. Consequently, the limited cable bandwidth had to be shared between power sources and data sources, further complicating the design and adjustment of the pulse-coupling networks, particularly whenever bi-polar pulses from two data sources were used.
These problems notwithstanding, even more information was imposed on the monocable. For example, casing-collar-locating pulses were applied to the cable, with their relatively long duration distinguishing them from other data sources. Similarly, commands were sent from the surface to subsurface equipment by varying a dc potential on the line. This latter technique was often used to switch between tools connected to the same cable to avoid the need for withdrawing the tool string completely from the hole to change tools whenever the required number of measurements caused the information capacity of the cable to be exceeded. However, inefficiently-used cable bandwidth was still limiting the flexibility and accuracy of the logging operation, causing the same formation to be logged several times in order to obtain all of the necessary information.
Other prior-art approaches to this problem involved analog frequency-modulation (FM) techniques and tone-burst commands. These approaches at least avoided inaccuracies resulting from unstable cable attenuations. However, both analog FM and tone bursts as they were used in the prior art made ony limited use of the full capability of the cable. Analog FM in particular had the usual stability, accuracy and dynamic-range problems of any analog telemetry system, which were compounded whenever several subcarriers handling different data sources were required.
In summary, the prior-art systems have been characterized by several disadvantages which were sufficiently severe to limit unnecessarily the quality and amount of data obtainable in the borehole-logging process. Many such systems were basically analog telemetry links, which were susceptable to the well-known problems in analog systems involving resolution, dynamic range, amplitude stability, signal distortion and noise. These problems were present in systems employing slowly-varying voltage or current signals, pulse-amplitude measurements or frequency-modulated carriers. Furthermore, even for data links which were basically pulse-counting systems that did not convey information by pulse amplitude, pulse-pileup effects and interferences between multiple data sources often limited instrument performance.
Although sometimes counting rates and frequencies could be chosen initially or controlled by signal processing to provide an acceptable data quality, severe limitations were often still present with regard to tool-pulling speed, the number of data sources which could be handled simultaneously, the range of parameters over which precise operation was possible, and the ability of the surface equipment to control subsurface operation. Prior-art techniques used to overcome these problems often resulted in the need for precise adjustments and data interpretation by expensive, well-trained personnel, basically marginal operation, or the need for expensive, hard-to-use multi-conductor cable.